1. Field of the Invention
The present invention generally relates to the regulation of supply voltages by switching regulators. More specifically, the present invention provides a pulse width modulation and burst modes switching regulator having automatic mode change.
2. Background Art
Switching regulators are used to regulate supply voltages for powering circuits. Specifically, supply voltages are regulated down or regulated up to appropriate voltage levels by a switching regulator.
Switching regulators can be broadly classified into fixed and non-fixed switching frequency types. Pulse width modulation (“PWM”) is typically used in the control loop for fixed frequency switchers. A high switching frequency is typically used to ensure low output ripple voltage. PWM switchers can achieve high power efficiency for moderate to high load currents. For low load currents, however, PWM switchers have low power efficiency due to the losses from high frequency switching. In the case of low load currents, a second class of non-PWM switchers, such as burst-mode (gated-oscillator) and pulse-frequency modulation (“PFM”) switchers, are used. The burst-mode switcher uses an oscillator of fixed-frequency and fixed duty-cycle clock which is gated by the feedback control loop to produce a lower average burst frequency at light loads. The PFM switcher uses a variable frequency clock that has either fixed ON-time or fixed OFF-time to control the switches. The second class of switchers is able to switch at a lower average frequency for low load currents. This enables it to have higher power efficiency for low load currents compared to PWM switchers.
A third class of switchers is the hysteretic self-oscillating type of switcher. The hysteretic self-oscillating switcher is basically an oscillator which generates its own switching frequency to control the ripples at its output. The hysteretic self-oscillating switcher has the advantage of being efficient to a larger load range, but suffers from poor switching frequency variation caused by external/internal component tolerances. Hysteretic self-oscillating switchers have a faster transient response compared to PWM switchers.
Many switching regulators often employ a combination of the first two classes of switchers mentioned above to cover the full load range. Accordingly, many switching regulators need to alternate between two different regulation loops or architectures to toggle between the PWM and non-PWM modes. Often, switching between modes must be made manually or physically. Depending on load current conditions, an external controller can choose the mode that will give higher power efficiency. However, for many applications, such as digital communications, it is not always possible to anticipate the load current condition and make mode changes for higher power efficiency. As a result, many such dual-mode switching regulators with separate PWM and non-PWM regulation loops are inefficient. What is needed is a switching regulator that can automatically toggle between PWM and non-PWM modes within the same regulation loop or architecture.